Dissolved-Cl2 triggered redox reaction enables high-performance perovskite solar cells

Constructing 2D/3D perovskite heterojunctions is effective for the surface passivation of perovskite solar cells (PSCs). However, previous reports that studying perovskite post-treatment only physically deposits 2D perovskite on the 3D perovskite, and the bulk 3D perovskite remains defective. Herein, we propose Cl2-dissolved chloroform as a multifunctional solvent for concurrently constructing 2D/3D perovskite heterojunction and inducing the secondary growth of the bulk grains. The mechanism of how Cl2 affects the performance of PSCs is clarified. Specifically, the dissolved Cl2 reacts with the 3D perovskite, leading to Cl/I ionic exchange and Ostwald ripening of the bulk grains. The generated Cl− further diffuses to passivate the bulk crystal and buried interface of PSCs. Hexylammonium bromide dissolved in the solvent reacts with the residual PbI2 to form 2D/3D heterojunctions on the surface. As a result, we achieved high-performance PSCs with a champion efficiency of 24.21% and substantially improved thermal, ambient, and operational stability.

The peak of PbI2 is too. Data in Supplementary Fig 9a reveals that Cl2-CF treatment increases the lattice parameter of FAPbI3 which is in contrast to the substitution of smaller Cl ions into I sites in FAPbI3. This is critical because it is the opposite of the main insist of this paper.
2. The authors insist that Cl2 plays an important role in the passivation and grain growth of FAPbI3 film which is confirmed by analysis in Figure 1. In addition, the authors insist that in the HABr/Cl2-CF treatment, Cl2 plays the same role as the Cl2-CF treatment. But as shown in Supplementary Fig6b, when HABr is dissolved in Cl2-CF, Cl2 produces Br2. If the state of Cl2 in the HABr/Cl2-CF solution is different from the Cl2-CF solution, the effect of Cl2 could not be interpreted by only diffusion into the bulk. There could be an effect of Br2.
3. Since this work fabricates FAPbI3 by sequential method, the control film includes a lot of PbI2. This is unusual. In most recent papers related to FAPbI3, the thin film surface in SEM images show clear without PbI2 which is in contrast to this work in Supplementary Fig. 11. In this paper, it is difficult to exclude the effect of the removal of PbI2 during post-treatment, making the argument of the paper unclear. Therefore, experiments on complete FAPbI3 thin films without PbI2 are essential to clarify the conclusion of this paper. Complete FAPbI3 thin films can be produced by either the two-step method [DOI: 10.1126/science.aaa9272, DOI: 10.1126 or by one-step method[https://doi.org/10.1038/nature14133].
Reviewer #3 (Remarks to the Author): Comments to the Author In this manuscript, authors report a new interface passivation strategy using solvent engineering method. Through Cl2-dissolved CF, authors provide the effective construction of the 2D/3D perovskite heterojunction. Also, this method enables high efficiency of over 24% with enhanced operational stability. Additionally, authors have performed appropriate analyzes to support their claims for reduced defect with this new passivation strategy. This new passivation strategy is obviously novel and different from the papers published so far. Thus, I recommend that this manuscript is published in Nature communications. Some aspects of the work may deserve the authors'further attention and then lead to revision: 1) Clearly, this strategy will give different passivation effects at grain and grain boundary. Therefore, 2D PL mapping analysis could further enhance the quality of this manuscript.
2) The grain size through SEM appears larger, but the FWMH of XRD does not seem to change significantly. Please compare the FWHM of the XRD to make sure the grain size of bulk is increased, not just the surface, and calculate the grain size from FWMH. of perovskite solar cells (PSCs). However, previous reports that studying perovskite post-treatment only physically deposits 2D perovskite on the 3D perovskite, and the bulk 3D perovskite remains defective. Herein, we propose Cl2-dissolved chloroform as a multifunctional solvent for concurrently constructing 2D/3D perovskite heterojunction and inducing the secondary growth of the bulk grains. The mechanism of how Cl2 affects the performance of PSCs is clarified. Specifically, the dissolved Cl2 reacts with the 3D perovskite, leading to Cl/I ionic exchange and Ostwald ripening of the bulk grains. The generated Clfurther diffuses to passivate the bulk crystal and buried interface of PSCs. Hexylammonium bromide dissolved in the novel solvent reacts with the residual PbI2 to form 2D/3D heterojunctions on the surface. As a result, we achieved high-performance PSCs with a champion efficiency of 24.21% and substantially improved thermal, ambient, and operational stability.
Regarding the efficiencies of the devices reported in this work, we are sure that the performance has a distinct gap compared with the world record (the record is 25.8% now). However, we think the devices still belong to the mainstream high-performance PSCs, and the reported device performance would not weaken the argument of the work.
Moreover, during the revision of the manuscript, the champion PCE of this work was updated to 24.21%. In the revised manuscript, the hysteresis of the device, the IPCE spectra, the steady-state power output, and the statistical PCE have been updated accordingly.
We sincerely invite the reviewer to reconsider our revised manuscript for publication in Nature Communications. for the PVSK-HABr/Cl2-CF device. Supplementary Fig. 23 shows the incident photon-to-current conversion efficiency (IPCE) in 300-900 nm. The integrated JSC for the PVSK, PVSK-HABr/CF, and PVSK-HABr/Cl2-CF was 24.78, 24.77, and 24.62 (mA cm -2 ), respectively. It is well matched with the value obtained from the J-V curves (<3% discrepancy), proving the reliability of the JSC results. As expected, along with the introduction of HABr/Cl2-CF, the hysteresis became negligible, which suggested that the defects were greatly passivated by HABr/Cl2-CF treatment ( Supplementary Fig. 26). Furthermore, the steady-state PCE measured at the maximum power point (Vmax) is shown in Supplementary Fig. 27. Compared with the other two devices, the PVSK-HABr/Cl2-CF device exhibited the most stable PCE of 23.30% at 0.99 V after measuring for 600 s.
Change 3: Finally, we achieve a high PCE of 24.21% for the optimized HABr/Cl2-CF device with negligible hysteresis effect.

Entry for the Table of Contents
We adopt Cl2-dissolved chloroform as a multifunctional and reactive solvent to construct 2D/3D perovskite heterojunctions. The redox reaction between Cl2 and Ileads to chloride doping, secondary growth of perovskite grains, and the formation of 2D/3D perovskite heterojunction. As a result, the device based on high-quality perovskite shows a champion efficiency of 24.21%. Comment 1. In the abstract, the authors claim that "previous reports only physically deposit a 2D perovskite passivation layer on the 3D perovskite layer. These methods are limited to surface passivation only, and the bulk 3D perovskite remains defective". I think this claim is not correct since several reports also experimented with adding 2D into the perovskite bulk and achieved some promising results.
Response: Thanks for this important comment. The previous claim was indeed not accurate and led to misunderstanding. We intended to express that previously reported post-treating methods were limited to depositing a physically stacked passivation layer on top of the 3D perovskite layer, but the bulk 3D perovskite couldn't be improved by these methods. The sayings in the last version seem to exclude the work that adding 2D into the perovskite bulk. Adding additives to the bulk of 3D perovskite and post-treatment by surface passivation are two important ways to improve the quality of perovskite films. The scientific issues in these two methods are quite different. This work focuses on developing a novel surface passivation method that concurrently passivates the surface defects and induces secondary crystal growth in the bulk 3D perovskite. Therefore, we restricted the claim in the abstract to "previous reports that studying perovskite post-treatment". The following changes were made in the revised manuscript.
Changes in the manuscript: Constructing 2D/3D perovskite heterojunctions is effective for the surface passivation of perovskite solar cells (PSCs). However, previous reports that studying perovskite post-treatment only physically deposits 2D perovskite on the 3D perovskite, and the bulk 3D perovskite remains defective.
Herein, we propose Cl2-dissolved chloroform as a multifunctional solvent for concurrently constructing 2D/3D perovskite heterojunction and inducing the secondary growth of the bulk grains. Comment 2. The XRD peak shift from 14.07 to 14.09 is very small? Is it within the resolution of the equipment?
Response: Thanks for the reviewer's comment. We consulted SmartLab's engineer and confirmed that the resolution of the X-ray diffractometer was 0.01 degree.
Therefore, the small shift from 14.07 to 14.09 was resolvable and meaningful. It was reported that a similar small shift of XRD peaks (about 0.01 to 0.02 degree) could be induced by adding different amounts of MACl into the precursor solution of 3D perovskites (Ye F, et al. Adv Mater, 2021, 33 2007126). Comment 3. It seems like the device performance is enhanced due to the synergetic effect of Cldiffusion and n-HABr passivation. What's about using n-HACl passivation? Would it have similar effect?
Response: Thanks for this constructive suggestion. We compared the passivation effect of n-HACl dissolved in pure chloroform without Cl2, n-HACl dissolved in Cl2dissolved chloroform, and n-HABr dissolved in Cl2-dissolved chloroform. As shown in Supplementary Fig. 24, n-HACl dissolved in fresh chloroform to a certain extent gave some performance improvement in VOC and PCE, which was most likely due to the synergetic effect of Cldiffusion and n-HACl passivation. However, n-HACl and n-HABr dissolved in Cl2-dissolved chloroform exhibited better average VOC and PCE than n-HACl dissolved in fresh chloroform, indicating that the dissolved Cl2 played an important role. This was most likely due to Cl2-induced secondary crystal growth that further reduced the defects in the bulk of the 3D perovskite.

Changes in the manuscript:
To further investigate the effect of Cl2, we compared the performance of the devices based on perovskite post-treatment by n-HACl dissolved in pure chloroform without Cl2, n-HACl dissolved in Cl2-dissolved chloroform, and n-HABr dissolved in Cl2-dissolved chloroform. As shown in Supplementary Fig. 24, n-HACl and n-HABr dissolved in Cl2-dissolved chloroform exhibited better average VOC and PCE than n-HACl dissolved in fresh chloroform, indicating that the dissolved Cl2 played an important role. This was most likely due to Cl2-induced secondary crystal growth that further reduced the defects in the bulk of the 3D perovskite. To expose and characterize the buried interface without damaging of the perovskite film, the previously reported technique was used (Yang, X. et al. Adv. Mater. 2021, 33, 2006435). Briefly, we first fabricated perovskite samples with the structure of ITO/PTAA/perovskite/Ag. And then the samples were immersed in chlorobenzene (CB) for 20 min in the N2-filled glovebox. After the PTAA layer was dissolved by CB, the perovskite/Ag film detached from the substrate and floated on the surface of CB.
Finally, the lift-off perovskite film was fixed to a base with its bottom side.

Changes in the manuscript:
Change 1: The corresponding energy dispersive spectroscopy (EDS) results in Fig.   2b revealed that the PVSK-Cl2-CF exhibited the highest Cl content of 2.64% at the bottom interface (vs. 0.28% for PVSK, 0.25% for PVSK-CF). Note that MACl was used to adjust the crystallization of perovskite, so a small amount of Cl was detected in the PVSK sample.

Methods
Calculation of Nt (Eω). The tDOS (Nt (Eω)) of the device was determined by measuring the impedance spectroscopy (EIS) and the Mott-Schottky curves in the dark using the previously reported method. 57 The Nt can be estimated by equation (4): where ω was the angular frequency, KB was the Boltzmann constant, T was the temperature, Vbi was the built-in electric field, e was the electron charge, W was the depletion width and C was the capacitance. The independent variable of energy Eω can be determined by equation (5): where βρ was the capture coefficient of hole, Nv was the effective density of states in the valence band. The depletion layer width W can be calculated by equation (6): where εs was the dielectric constant of perovskite active layer, NA and ND were the doping concentrations of the hole-transporting layer and the electron-transporting layer respectively. Vbi can be determined by measuring the Mott-Schottky curve and calculated by equation (7): where n was a constant of proportionality. Then the formula for k was expressed as: Therefore, equation (6) was simplified to (9).
= × √− 2 × (9) In addition, the geometric capacitance Cg can be obtained from the high-frequency region of the EIS data by equation (10): where d was the thickness of the perovskite layer, then equation (3) can again be expressed as:  Fig. 28). After 1920 hours of aging in the air with a relative humidity of 10% and a temperature of ~25°C , the unencapsulated device based on PVSK-HABr/Cl2-CF maintained 91% of its original PCE while the device based on PVSK and PVSK-HABr/CF maintained 78% and 82% respectively. Supplementary Fig. 29 and Supplementary Fig. 30 showed that HABr/Cl2-CF treatment improved the thermal stability with the devices based on PVSK-HABr/Cl2-CF remained over 92% of the initial PCE after heating for 400 hours at 55 °C in a nitrogen environment and remained over 81% of the initial PCE after heating for 106 hours at 85°C in a nitrogen environment.

Reviewer #2
This paper reports that chloroform is oxidized to form Cl2, and the Cl2 formed in this way is treated on the surface of perovskite to penetrate not only the surface but also the bulk, effectively passivating defects, and growing crystal grains. The approach is interesting and the influence of Cl2 is clear from the experimental results, but there are critical concerns about reaching conclusions. I believe that only when these concerns are addressed can we decide whether to publish the paper.
General response: We sincerely thank the reviewer for making many valuable comments for further improving the quality of our manuscript. We have comprehensively revised the manuscript according to the reviewer's suggestions. This is critical because it is the opposite of the main insist of this paper.

Response:
Thanks for the reviewer's important comment. We have re-checked the related XRD data and repeated the XRD experiment. We found that we made a mistake by using the wrong data in Supplementary Fig. 9 in the previous version.
The updated XRD results showed that Cl2-CF treatment can induce a shift of the XRD peak of α-FAPbI3 from 14.00° to 14.05° and the shift of the PbI2 peak from 12.70° to 12.75°. This was due to the substitution of smaller Clions into Isites, which led to the lattice contraction of the perovskite crystal.

Changes in the manuscript:
We performed X-ray diffraction (XRD) measurements to study the effect of Cl2-CF on the crystal structure of perovskite. After the Cl2-CF treatment, compared to PVSK, PVSK-Cl2-CF exhibited higher XRD peak intensity, indicating improved crystallization (Supplementary Fig. 9a). Due to the ionic exchange of Iand Cl -, XRD patterns of α-FAPbI3 located at 14.00° shifted to 14.05° in PVSK-Cl2-CF, indicating the formation of α-FAPbI(3-x)Clx (Supplementary Fig.   9b). Similarly, the XRD peak of the residual PbI2 revealed a peak shift from 12.70° to 12.75°, indicating the formation of PbI(2-x)Clx (Supplementary Fig. 9c).
Comment 2. The authors insist that Cl2 plays an important role in the passivation and grain growth of FAPbI3 film which is confirmed by analysis in Figure 1. In addition, the authors insist that in the HABr/Cl2-CF treatment, Cl2 plays the same role as the We first studied the state of Cl2 in the HABr/Cl2-CF solution. As shown in Supplementary Fig. 16, when placed the wet starch potassium iodide test paper above the HABr/Cl2-CF solution, it turned blue immediately. This result indicated that although partial Cl2 took part in oxidizing the Brions of HABr, there was residual Cl2 in the solution.
Therefore, compared to the Cl2-CF solvent, Cl2 in the HABr/Cl2-CF solution can be divided into two parts. One part took part in oxidizing Brions to Br2 and leaving Clions in the solution. The other part was the residual Cl2 in Cl2-CF solvent, which would penetrate the depth of perovskite films to trigger the redox reaction with perovskite, inducing the Cl-doping and secondary growth of perovskite crystal grains.
During the post-treatment experiment using HABr/Cl2-CF solution, we observed a light-yellow color for the as-treated perovskite films (Supplementary Fig. 17 Comment 3. Since this work fabricates FAPbI3 by sequential method, the control film includes a lot of PbI2. This is unusual. In most recent papers related to FAPbI3, the thin film surface in SEM images show clear without PbI2 which is in contrast to this work in Supplementary Fig. 11. In this paper, it is difficult to exclude the effect of the removal of PbI2 during post-treatment, making the argument of the paper unclear. Therefore, experiments on complete FAPbI3 thin films without PbI2 are essential to clarify the conclusion of this paper. Complete FAPbI3 thin films can be produced by In our group, the composition and depositing conditions of the control perovskite were optimized in the previously published works (Nat. Commun. 2022, 13, 4891;Adv. Energy Mater. 2023, 13, 2204362;J. Mater. Chem. A. 2021, 9, 20807). So we continued to use this optimized control perovskite with a small amount of residual PbI2 in this work. We agree with the reviewer that PbI2-contained perovskite may to a certain extent complicate studying the effect of Cl2. However, we think that starting from the optimized control with higher device performance will make the work more attractive. Moreover, due to the passivation effect of the residual PbI2, it is reasonable to argue that the removal of PbI2 during post-treatment will most likely gave a deleterious effect. So this aspect will perhaps not weaken the arguments regarding the role of Cl2 in the improvement of device performance in this work.
According to the reviewer's suggestion, we also studied the effect of HABr/Cl2-CF treatment on the PbI2-free (or PbI2-less) perovskite. As shown in the SEM images in Supplementary Fig. 12, no distinct secondary growth of crystal grains can be observed. This result indicated that during the treatment by the HABr/Cl2-CF solution, Cl2 was most likely first reacted with the residual PbI2 and this reaction triggered the subsequent secondary grain growth, leading to high-quality perovskite films with larger grains.

Supplementary Fig. 12 | SEM images of perovskite films without and with
HABr/Cl2-CF treatment. The perovskite films were fabricated by the one-step spincoating method and showed almost a PbI2-free surface.

Changes in the manuscript:
The roughness of the PVSK-HABr/Cl2-CF is largely reduced to 26.2 nm, which was comparable with that of 25.7 nm for the PVSK-HABr/CF and much less than that of 32.9 nm for the PVSK (Supplementary Fig. 11 g-i). The effect of HABr/Cl2-CF treatment on the PbI2 free (or PbI2 less) was also studied. As shown in the SEM images in Supplementary Fig. 12, no distinct secondary growth of crystal grains can be observed. This result indicated that during the treatment by the HABr/Cl2-CF solution, Cl2 was most likely first reacted with the residual PbI2 and this reaction triggered the subsequent secondary grain growth, leading to high-quality perovskite films with larger grains.

Reviewer #3
In this manuscript, authors report a new interface passivation strategy using solvent engineering method. Through Cl2-dissolved CF, authors provide the effective construction of the 2D/3D perovskite heterojunction. Also, this method enables high efficiency of over 24% with enhanced operational stability. Additionally, authors have performed appropriate analyzes to support their claims for reduced defect with this new passivation strategy. This new passivation strategy is obviously novel and different from the papers published so far. Thus, I recommend that this manuscript is published in Nature communications. Some aspects of the work may deserve the authors' further attention and then lead to revision: General response: We thank the reviewer for reviewing the manuscript and giving valuable comments. We have carefully revised the manuscript following the reviewer's suggestions.
Comment 1. Clearly, this strategy will give different passivation effects at grain and grain boundary. Therefore, 2D PL mapping analysis could further enhance the quality of this manuscript.
Response: Thanks for this constructive comment. We performed 2D PL mapping measurements and the images were shown in Supplementary Fig. 18. The results showed that compared with the PVSK and PVSK-HABr/CF samples, the PVSK-HABr/Cl2-CF sample showed overall higher PL intensity as well as larger perovskite grains, indicating better passivation effect and secondary grain growth. Moreover, there were a lot of dark grain boundaries for the PVSK and PVSK-HABr/CF samples while most of them disappeared in the PVSK-HABr/Cl2-CF sample. These results indicated that the HABr/Cl2-CF treatment led to a superior passivation effect at grain boundaries.

Changes in the manuscript:
Change 1: The 2D PL mapping images shown in Supplementary Fig. 19 demonstrated that PVSK-HABr/Cl2-CF showed overall higher PL intensity as well as larger perovskite grains, indicating better passivation effect and second grain growth.
Moreover, there were a lot of dark grain boundaries for the PVSK and PVSK-HABr/CF samples while most of them disappeared in the PVSK-HABr/Cl2-CF sample. These results indicated that the HABr/Cl2-CF treatment led to a superior passivation effect at grain boundaries.
Change 2: Characterization. PL mapping images were acquired using a laser confocal microscope (Leica TCS SP8) excited at a 488 nm pulse.
Comment 2. The grain size through SEM appears larger, but the FWMH of XRD does not seem to change significantly. Please compare the FWHM of the XRD to make sure the grain size of bulk is increased, not just the surface, and calculate the grain size from FWMH.

Response:
Thanks for the reviewer's important suggestion. We compared the grain size of the bulk using the FWHM of the XRD. The relationship between the grain size and the FWHM of the XRD peak can be expressed by the Scherrer equation: where D is the mean crystallite size, K is the Scherrer constant, λ is the wavelength of the incident X-ray, FWHM is the full-width at half-maximum of the XRD peak (with instrumental broadening removed), and θ is the Bragg angle.  As shown in Supplementary Fig. 10, the SEM image of the bottom side of the perovskite film also showed increased grain size for the PVSK-HABr/Cl2-CF, which indicated that the grain growth was not only occurred at the surface of the film but in the bulk of the whole film.
Comment 3. This manuscript employs HABr to form 2D/3D structures. Therefore, the papers introducing HABr for the first time should be cited (DOI: 10.1039/C9EE00751B) and explained with its advantages.

Response: Thanks very much for this important comment. Yoo et al. first utilized
HABr as the 2D perovskite precursor to construct the 2D/3D structure (Energy Environ. Sci. 2019, 12, 2192). They found that HABr can be selectively dissolved in chloroform (CF), which was non-solvent for perovskite films. Therefore, treating the 3D perovskite films with HABr/CF could construct a 2D perovskite layer without damage that was commonly observed in the post-treatment by using alkyl ammonium salt dissolved in IPA. Therefore, this strategy can help fabricate high-performance perovskite solar cells with improved stability.

Changes in the manuscript:
Change 1: The most widely used method for constructing 2D/3D heterojunctions is spin-coating bulky alkyl ammonium halides onto the 3D perovskite, followed by a thermal annealing process. For example, Yo o et al. first utilized hexylammonium hydrobromide (HABr) as the 2D perovskite precursor to construct the 2D/3D structure. 31 They found that HABr can be selectively dissolved in chloroform (CF), which was a non-solvent for perovskite films. Therefore, treating the 3D perovskite films with HABr/CF could construct a 2D perovskite layer without damage that was commonly observed in the post-treatment by using alkyl ammonium salt dissolved in isopropanol (IPA). Therefore, this strategy can help fabricate high-performance perovskite solar cells with improved stability. However, the resultant 2D perovskite capping layer in previous reports was most likely physically stacked onto the 3D perovskite, and only weak interfacial interactions existed between the 2D and 3D perovskite layers.
Change 2: Herein, we report adopting chlorine-dissolved chloroform (Cl2-CF) as a multifunctional solvent for selectively dissolving HABr to construct 2D/3D perovskite heterojunction, as well as to induce secondary growth of perovskite grains and defect passivation through the redox reaction between Cl2 and I -.